treewide: Convert CONFIG_HOSTNAME to a string option

CONFIG_HOSTNAME is defined as a "plain" preprocessor string, but every
use is couched by __stringify(...).

Hence, convert it to a proper string option.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
diff --git a/include/configs/x86-common.h b/include/configs/x86-common.h
index 7cf5c27..6d650b7 100644
--- a/include/configs/x86-common.h
+++ b/include/configs/x86-common.h
@@ -110,7 +110,7 @@
 
 /* Default environment */
 #define CONFIG_ROOTPATH		"/opt/nfsroot"
-#define CONFIG_HOSTNAME		x86
+#define CONFIG_HOSTNAME		"x86"
 #define CONFIG_BOOTFILE		"bzImage"
 #define CONFIG_LOADADDR		0x1000000
 #define CONFIG_RAMDISK_ADDR	0x4000000